The present invention relates to braking systems for vehicles, and more particularly to antilock braking systems for vehicles.
Stopping a car quickly on a slippery road is challenging task for drivers. Anti-lock braking systems (ABS) have significantly improved stopping distances on both dry and slippery roads. On slippery roads, even professional drivers cannot stop a vehicle as quickly without ABS as an average driver can with ABS. ABS rely on the fact that a skidding wheel (where the tire contact patch is sliding relative to the road) of a vehicle has less traction than a non-skidding wheel. By preventing the wheels from skidding while the vehicle slows down, ABS operation stops the vehicle faster and allow the vehicle to be steered during a panic stop.
Most ABS do not include costly brake pressure transducers, brake pedal force sensors, or attitude sensors. Conventional ABS include a xe2x80x9cpeak seekingxe2x80x9d control method that slowly adjusts the wheel slip and wheel deceleration thresholds by applying rate controlled brake pressure increases. This peak seeking method may require several apply and release cycles to approach the correct slip and deceleration target thresholds. The adjustments made by these systems usually happen too slowly to adjust for inclines, turns, and/or vehicle loading.
Normal front to rear weight distribution of a front wheel drive (FWD) vehicle at rest is 60% front and 40% rear (60/40). Left to right balance is typically 50/50. Weight transfer due to braking at 0.9 g on a level surface in a straight line creates a front to rear weight distribution of 84/16. Weight transference due to a right turn at 0.3 g on a level surface at a steady speed creates a left to right weight distribution of 65/35. Pitch and roll angles of the road surface further complicate these dynamics.
Conventional ABS do not compensate for changes in the distribution of vehicle weight when braking or turning a corner. Conventional ABS also do not compensate for changes in weight distribution and traction when additional items are added to vehicle storage compartments. For example, the driver may load a trunk of a vehicle or a bed of a pickup truck with heavy items such as luggage or loads such as gravel. Alternately, an interior compartment of the vehicle may be filled with passengers and/or other heavy items. The failure to adequately compensate for the additional vehicle weight may cause instability during braking.
For example, when braking on a curve while traveling downhill, conventional ABS fail to optimize available traction to decelerate the vehicle as quickly as possible. Conventional ABS fail to account for changing steering angle and its impact on vehicle weight distribution and traction. A rear wheel located on an inner side of the downhill turn has dramatically reduced weight and traction. The front wheel on an outer side of the downhill turn has increased weight and traction. Vehicle control may be adversely impacted if too much brake torque is applied while engaging the brakes in a turn, particularly in a turn on a downgrade. Vehicle load is another factor that can dramatically impact ABS performance.
A method and apparatus according to the present invention for operating a vehicle anti-lock braking system includes a brake pedal and a brake modulator. The anti-lock braking system reduces braking pressure by an initial pressure reduction after detecting insipient wheel lock. Vehicle deceleration is measured as a function of brake pedal position. A first table is updated with the vehicle deceleration and the brake pedal position. A coefficient of friction of a road surface is estimated based on the first table. A slip target for a wheel is generated based on an estimated maximum slip of the wheel before losing traction minus an estimated potential for vehicle rollover.
In other features of the invention, a deceleration target for the wheel is generated based on an estimated maximum deceleration of the wheel before losing traction minus the estimated potential for vehicle rollover.
In still other features, rollover lateral acceleration is estimated. The estimated potential for vehicle rollover is based on the estimated rollover lateral acceleration. Grade is estimated based on rolling resistance, drag, engine braking, brake torque and acceleration. Vehicle weight, steering angle and steering rate are estimated.
In other features, weight distribution for the wheel is estimated. The weight distribution is adjusted for roll, pitch, and lateral and longitudinal acceleration. Brake release and apply torque for each of the wheels is calculated based on the coefficient of friction, attitude, and weight distribution.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.